If a dog gains weight for no apparent reason and its diet has not changed, it is possible the canine may be developing a thyroid problem. Hypothyroidism occurs when the thyroid becomes underactive, and the illness is typically caused by one of two underlying diseases - lymphocytic thyroiditis or idiopathic thyroid gland atrophy - according to VCA Animal Hospitals.

A dog with an underactive thyroid gland may also display clinical signs such as lethargy and lack of motivation, a thinning coat, excessive shedding and darkened skin pigmentation. VCA reports that when the thyroid becomes underactive, a dog’s metabolism slows down, which has a detrimental effect on nearly every part of the dog's body. Increases in a dog's cholesterol levels are associated with about 75 percent of canines with the illness.

The diagnosis of hypothyroidism is made based on the classic clinical signs, as well as a simple blood tests and a specialized thyroid panel. While there is no cure for hypothyroidism, a veterinarian will readily be able to treat the disease with hormone supplementation. If you notice any of the clinical signs of hypothyroidism, you should bring your canine companion to a pet health professional as soon as possible.

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